Missing channels are a classic symptom of poor signal strength. [NB: The strength and quality of the signals arriving at your aerial don't remain constant. They're affected by lots of factors, including things like the high atmospheric pressure we're currently encountering, which can make it easier for interference from other TV regions (or from abroad) to occur. So getting adequate signal strength on one day doesn't mean that you'll always get it on other days].
The first thing to do is to take a look at your TV aerial and check that there's no obvious problem with it. (I offered that same advice to someone on AB once and they came back to say that they'd found their TV aerial lying on the ground in their back garden! You might find that the storms we had earlier in the year have knocked your aerial out of alignment).
Then check that any plugs and connections in the aerial's lead are all secure. (Simply not having the aerial plug pushed firmly enough into the TV's socket can cause problems).
If that doesn't help, simply wait for a day or two to see if the problem clears itself. (High pressure problems yesterday meant that the normally strong signal I get on my radio from Classic FM suddenly disappeared under much stronger interference from a Belgian station. When similar interference problems occur with Freeview signals you tend not to see or hear the interfering stations but you simply lose channels instead. So your problem might disappear - possibly after another retune - if you simply wait a while).
If things still don't improve, install a signal booster
http://www.argos.co.uk/product/5713543
(You'll need a fly lead to go with it:
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720 )